The present invention relates to a process and a composition for promoting gas hydrate formation. The invention also relates to a process for storing gas using gas hydrates. This invention was developed as a result of a contract with the United States Department of Energy.
Current means of storing natural gas (i.e., gas compositions constituted primarily of methane but that may contain minor amounts of other components such as ethane, propane, isobutane, butane, and/or nitrogen) or any of its components include, for example, compressed gas storage, liquified gas storage, underground storage, and adsorption. Each of these means of storage, however, have undesirable deficiencies. For example, liquified gas storage involves high costs and hazards such as the possibility of the gas tank rupturing. Underground storage of natural gas is limited to the regions of the country with satisfactory geological features such as porous sandstone formations and salt domes. Such geographical features are not usually found in most populous regions where the demand for natural gas is greatest; therefore, the gas must be stored where these geographical features are found and then shipped to where it is needed. Compressed gas storage, like liquified gas storage, involves high costs and hazards primarily because of the high pressures involved in storing gas in this manner.
A search for alternative methods of storing natural gases has led to the consideration of clathrates. Clathrates are distinguished by having molecules of one type completely enclosed within the crystalline structure of molecules of another type. Gas hydrates are a subset of the class of the solid compounds called clathrates. Gas hydrates are crystalline inclusion compounds formed when water and gas are mixed under conditions of elevated pressure and reduced temperatures. Through hydrogen bonding, the host water molecules form a lattice structure resembling a cage. For gas hydrates, a guest molecule such as, for example, natural gas and its components, is contained within the cage-like crystalline structure of the host water molecule.
Utilizing gas hydrates as a means to store natural gas or its components has not been practical because of numerous deficiencies. First, the formation of hydrates in a quiescent system is extremely slow at hydrate-forming temperatures and pressures. The typical mechanism of hydrate formation in a quiescent pure water-gas system is as follows: water molecules first form clusters by hydrogen bonding in the liquid phase, proceeding to cluster and occlude gas until a critical concentration and size of the clusters is reached. This is the critical nuclei for hydrate formation. After an induction time of about 20 minutes, depending upon system conditions (i.e., temperature and pressure), particle agglomeration of these nuclei proceeds at the water-gas interface, resulting in the formation of a thin film of hydrates on the surface that isolates the water from the gas, thereby drastically slowing the rate of hydrate formation because the water and the gas must then diffuse through the thin film to perpetuate hydrate growth. Attempts to improve hydrate formation rate in a quiescent system include both a xe2x80x9crocking cellxe2x80x9d apparatus in which the rocking motion establishes enough turbulence to periodically sweep away the hydrate film that forms on the water surface preventing contact with the gas and mechanical stirring to generate renewed surface area of the water in contact with the gas. The generation of renewed surface area via a rocking motion or mechanical stirring is necessary for rapid hydrate formation because otherwise the thin film of hydrates on the surface of the water isolates the gas from the water. This decreases the rate of gas absorption into the free water and drastically slows the formation of hydrates.
Another deficiency in establishing a practical means of gas storage using hydrates results from the entrapment of free water (i.e., water not bound in hydrate form) between hydrate particles. The solid mass of hydrates includes a large amount of water entrapped between hydrate particles and isolated from the gas. Typically, more water is trapped between solid hydrate particles than is bound in the hydrate structure. The appreciable volume of storage space occupied by this entrapped interstitial water means that much of the storage space is occupied by water not containing gas. The entrapment of free water between solid hydrate particles has been a deterrent to practical use of hydrates because the result is inefficient packing of the gas which, in turn, results in a low storage capacity for the gas. Even when the hydrates are created by mechanical stirring, the entrapped water still represents a large percentage of the volume.
Yet another deficiency in using hydrates for storing gas is the complexity of the hydrate formation-storage-decomposition process. Typically, a water-hydrate slurry forms as the hydrates develop. The thickness of the slurry makes mechanical stirring difficult. Also, the hydrate particles grow in a random pattern within a formation vessel and must be removed from the slurry and packed in a separate container for storage. This separation and packaging step requires an often difficult and economically unfeasible mechanical process.
In view of the aforementioned deficiencies attendant with the prior art methods, it is clear that there exists a need in the art for practical methods of utilizing hydrates as a means of storing gas and for the corresponding compositions.
Accordingly, one object of this invention is to provide a practical and economically-viable means for storing gases such as, for example, natural gas and its components, using hydrates.
Another object of the invention is to simplify the process of gas hydrate formation and storage.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention as embodied and broadly described herein, there is provided a method of storing gas comprising forming gas hydrates in the presence of a water-surfactant solution.
To further achieve the foregoing and other objects, this invention is also directed to a composition for promoting gas hydrate formation comprising a mixture of water, an effective amount of a surfactant and at least one hydrate-forming constituent.